Generally, an internal combustion engine for a vehicle includes a fuel vapor treatment apparatus. The fuel vapor treatment apparatus restricts fuel vapor, which is produced in a fuel tank, from diffusing into the atmosphere. Fuel vapor is introduced from the fuel tank into a canister of the fuel vapor treatment apparatus. The canister accommodates an adsorbent to temporarily adsorb the fuel vapor into the adsorbent. The fuel vapor adsorbed into the adsorbent is desorbed from the adsorbent by negative pressure in the intake pipe of the engine, so that fuel vapor is purged into the intake pipe through a purge pipe in an engine operation. Fuel vapor is desorbed from the adsorbent, so that the absorptivity of the adsorbent recovers.
When fuel vapor is being purged from the canister, the air/fuel ratio of mixture introduced into the engine needs to be controlled at a target air/fuel ratio such as a theoretical air/fuel ratio, in general. For this purpose, the flow rate of the mixture needs to be controlled at an appropriate value. The flow rate of the mixture can be determined on the basis of a state of the mixture such as a concentration (fuel vapor concentration) of fuel vapor in the mixture. Accordingly, the fuel vapor treatment apparatus includes a unit for determining the fuel state of the mixture.
For example, a fuel vapor treatment apparatus disclosed in JP-A-7-269419 includes an air/fuel ratio sensor provided to the exhaust pipe of the engine for measuring the air/fuel ratio. The fuel vapor concentration indicating the fuel state of the mixture is determined on the basis of the difference between the target air/fuel ratio and the air/fuel ratio measured using the air/fuel ratio sensor.
Volatility of fuel variously changes depending upon a season, a region, and the like. When a fuel injection amount is set without considering discrepancy of a fuel property indicating different volatility, a fuel amount injected into the engine becomes excessive or deficient. Therefore, a unit for determining the fuel property is preferably added to the fuel vapor treatment apparatus. However, it is costly to additionally provide a unit for determining the fuel property.
A generally known leakage detecting apparatus includes a fuel tank and a canister. The leakage detecting apparatus therein defines an enclosed cavity including a space in which fuel vapor is purged into the intake pipe of the engine. The leakage detecting apparatus detects a leaking hole greater than a predetermined size in the enclosed cavity on the basis of change in pressure in the enclosed cavity.
In the leakage detecting apparatus, an erroneous determination may occur when the fuel property is not considered. For example, gas is drawn from the enclosed cavity using a pump of a leakage detecting apparatus. Thereafter, when pressure after a predetermined period is greater than reference value, a leaking hole is determined to exist because of inflow of the external air through the leaking hole. In such an apparatus, when fuel is highly volatile, pressure in the interior of the enclosed cavity is apt to become greater. Accordingly, even when a leaking hole does not exist, the pressure in the enclosed cavity may become greater than the reference value after the predetermined period.
In addition, in an injection control apparatus, the fuel injection amount is preferably controlled by determining the fuel property at low cost.